Acta Meteorol. Sinica, 9, 169-183

Jiping CHAO

National Research Center for Marine Environment Forecasts, Beijing

Yonghui LIN

Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing

Bin WANG

Department of Meteorology, University of Hawaii at Manoa, Hawaii

(Received August 18, 1994, revised October 20, 1994)
ABSTRACT

In this paper, a tropical atmospheric model of relevance to short-term climate variations (Wang and Li 1993) is utilized for study of the development of Madden-Julian oscillation. The model contains an interactive process of boundary-layer Ekman convergence and precipitation heating. The model is solved by expanding dependent variables in terms of parabolic cylindrical functions in the meridional direction and truncating three meridional modes n=0,2,4 for equatorial symmetric solutions. The free wave solutions obtained under long-wave approximation are induced as a Kelvin wave and two Rossby waves. After considering the effect of boundary-layer dynamic process, the modified Kelvin wave becomes unstable in long-wave bands with a typical growth rate in an order of 10-6 s-1 and an eastward phase speed of 10 m s-1; the most unstable mode is wavenumber one. These theoretical results are consistent with the observed Madden-Julian oscillation in equatorial area. For the two modified Rossby waves, one with a smaller meridional scale (n=4) decays except for extra long-waves; the other with a larger meridional scale (n=2) grows in short-wave bands. This may be relevant to explaining the westward propagation of super cloud clusters in the Madden-Julian oscillation. The theory suggests that the boundary-layer dynamic process is an important mechanism in the development of the Madden-Julian oscillation.